This invention relates to wireless electronic information transmitters, and more particularly to wireless transmitters which transmit information on a time division basis. Basically, a wireless transmitter transmits the information signals via electromagnetic radiation. These radiated signals may be amplitude modulated, frequency modulated, single-sideband modulated, or pulse position modulated as an example. A variety of circuits have been described in the prior art for performing this modulation. See for example, a book entitled, Information Transmission, Modulation, and Noise by Mischa Schwartz published by McGraw-Hill in 1959.
Since wireless transmitters use electromagnetic radiation to transmit their signals, the electromagnetic frequency spectrum must be divided up in a manner which enables multiple users to transmit simultaneously. Presently, one method of accomplishing this is to assign specific frequency bands to each of the multiple users. However, a problem with this method is that to allow more users to transmit requires that the width of the bands be reduced. As a result, the maximum frequency of the information signals which each user may transmit is also reduced. Further, as the number of frequency bands increase, the circuitry required to filter one band from the adjacent bands becomes more complex.
Another method for enabling multiple users to transmit simultaneously is to assign one frequency band to the multiple users and to time share its use. In this method, each user samples their information signals; and the samples of all the users are transmitted in an interleaved manner such that none of the samples overlap in time. A problem however, with the time division transmitters of the prior art is that the sampling signals themselves have an undesirably wide frequency spectrum. As a result, the width of the one time shared frequency band is also undesirably large. In some systems the ratio of number of users to frequency bandwidth for the time shared transmit method is less than the ratio of number of users to total bandwidth used by a channel assignment transmit method. See for example pages 160-163 of the above cited reference wherein the frequency spectrum of square wave sampled information signals is described. This spectrum equals the frequency spectrum of the unsampled information signals centered about multiples of the sampling frequency, with each multiple having a sine x/x weighting. Accordingly in that system, the frequency spectrum of the sampled signals extended out to the frequency at which the amplitude of the sine x/x function became sufficiently small itself becomes excessive.
Therefore, it is one object of the invention to provide an improved time division transmitter.
It is another object of the invention to provide a time division transmitter having sampling signals of a highly controllable bandwidth.
It is another object of the invention to provide a time division transmitter having specially shaped sampling signals which are physically realizable with a surface wave device.
Still another object of the invention is to provide a wireless time division portable telephone which is suitable for coupling to a telephone system.